Ground fault interrupter systems are intended to sense small differences in current in normally balanced power lines. These differences may be caused by leakages of current from one of the line conductors to ground, thus depriving the neutral line of some of its normal current which could establish a balance or zero difference in curent in the lines at the sensor. If the differential currents are below predetermined levels, power should normally be allowed to flow uninterrupted. If larger differential current occur, the circuit should be interrupted, since it is then probable that a malfunction of insulation or perhaps even a serious shock to a human being is occurring. Some such ground fault interrupter systems are described and claimed in my U.S. Pat. No. 3614534, issued Oct. 19, 1971 on improvements in "Ground Fault Responsive Electrical Protective Systems."
In practical field use of ground fault interrupters it is possible for spurious signals to be confused with real fault currents. For example, power line transients due to sudden load changes or to lightning-induced surges can give rise to nuisance tripping in ground fault interrupter systems. In tolerance to frequent nuisance tripping can cause the users of such equipment to establish sensitivity specifications at dangerously high levels. A steady-state spurious signal frequently encountered is a capacitive current from the high line side to ground. This can be caused by a long buried cable or by discrete capacitors used to avoid radio frequency interference or by similar circuit influences having nothing to do with a fault on the line. The minimization of nuisance tripping in ground fault interrupter systems by discriminating against reaction currents, spurious transients or other electrical signals having a wave form which does not correspond to or correlate in phase with the line voltage is addressed in another of my U.S. Pat. No. 3723814, issued Mar. 27 1973 and entitled "Phase-Sensitive Ground Fault Protective Systems."
The present invention is also concerned with the problem of nuisance tripping in ground fault protective systems. The term "nuisance tripping" implies, of course, that interruption of the circuit is brought about by the ground fault detector and interrupter system for causes which prove to be insufficient, that the system responds to electrical conditions by needlessly breaking the circuit without the occurence of a true fault. A true ground fault can have different causes and can give rise to different levels of current imbalance in the supply conductors. If the current imbalance is comparatively high, that is to say, if a comparatively large ground fault current flows, the system should respond quickly and decisively. But if the current imbalance in the supply lines is comparatively small, it is not necessarily desirable for the system to respond as quickly to the signal condition. Under such circumstances the decision to interrupt the supply circuit can be deferred, in a manner of speaking, to permit the system more time to test whether the current imbalance in the supply conductors is a short-term anomaly or a longer term effect representing an actual or incipient ground fault.
Despite the belief by some that a ground fault system should interrupt with a uniform time delay regardless of the magnitude of the fault current, it can be demonstrated than an ideal ground fault system designed to operate safely with a minimum of nuisance tripping should respond with different time delays, depending upon the magnitude of the differential currents in the supply conductors. Ideally, the response time should have an antilog characteristic with respect to the fault- characterizing signals derived from the supply lines. To provide such a system is among the objectives of the present invention.
One embodiment of the present invention achieves this objective by augmenting of expanding ground fault signals at higher signal levels and integrating th expanded signal at rates which are disproportionately higher for higher signal levels than for lower signal levels. Interruption of the supply circuit is in response to the expanded and integrated ground fault signals. Another embodiment achieves this same objective by means of a specially adapted integrator giving an enhanced response to strong signals of short duration. Such an integrator in combination with a succeeding threshold device provides a similarly augmented and faster response to higher signal levels than to lower signal levels.